1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
In general, gradation stability is very important for a printer. A conventional image forming apparatus of an electrophotographic system secures the gradation stability by measuring a toner density on an image carrier such as a photoconductive drum and an intermediate transfer belt to execute a gradation correction based on a measurement result thereof. However, since this method does not consider influences of image transfer (secondary transfer) and fixation from the intermediate transfer belt to a sheet, the gradation stability cannot be ensured on an actual output sheet.
There has been therefore used a method for executing gradation correction of a printer by forming a gradation pattern image including plural density patches on a sheet to measure the gradation pattern image on the output sheet by a density sensor (see Japanese Patent Application Laid-Open Publication No. 2005-167550). Because of using the actual output sheet to execute the gradation correction every predetermined interval, a stable output result can be obtained.
However, since the method which forms the gradation pattern image on the sheet needs to actually output the sheet, a waste sheet is generated every time correction is performed. Because limited number of patches can be formed on one sheet, when using a pattern of 32 gradations for example, outputting gradation patterns by using four sheets of A4 size becomes necessary (see FIG. 4).
Especially in Production Print (PP) field, constant image qualities are required for providing prints as commercial products, but outputting a volume of waste paper is not preferable.